Optical defect detection technology has been one of the key technologies limiting our ability to make ever smaller transistors. It has, up till now, provided both high performance and high throughput, which other technologies like electron beam microscopy could not offer. However, as the geometries employed in IC chips have continued to decrease, it has become harder to detect defects reliably. Design rules of future generations of IC chips are so small that there is a real possibility that none of the current optical defect detection technologies will work. Therefore, in order to extend the life of optical inspection technology into future equipment generations, a major overhaul of optical defect detection technology is needed.
Optical defect detection systems in use today include both bright field systems and dark field systems. Unlike bright field systems, dark field systems attempt to exclude the unscattered illumination beam from the image. However, limitations of the current dark field and bright field defect detection systems exist which cause difficulty in reliably detecting defects, especially as the design rules progressively decrease. Separate path interferometric techniques have been proposed according to which two beams, probe and reference beams, are generated using a beam splitter and brought to an image sensor through different paths or subsystems. For example, separate path systems designed for defect detection are discussed in U.S. Pat. Nos. 7,061,625, 7,095,507, 7,209,239 and 7,259,869. These and the other patents identified in this patent specification, as well as all non-patent references identified in this patent specification, are hereby incorporated by reference. Another separate path system which is designed for high resolution surface profiling is the Linnik interferometer (see, M. Francon, “Optical Interferometry,” Academic Press, New York and London, 1966, p 289.) These separate path interferometric systems are, in principle, capable of amplifying the defect signal or measuring both the amplitude and phase of the defect signal. However, these systems are not only complex and expensive but also have serious drawbacks; photon noise and sample pattern noise can be excessive and also they are unstable due to the two different paths the probe and reference beams take. Small environmental perturbations like floor vibrations, acoustic disturbances, temperature gradients, etc., can easily destabilize the system. Consequently, it is difficult to use this kind of separate path interferometric system in industrial environments.
Conventional phase-contrast microscopes are designed to provide a fixed amount of phase control to specular component, usually π/2 or −π/2. These systems commonly use extended light sources such as an arc or halogen lamp. Although they are generally suitable for observing biological samples, conventional phase-contrast microscopes are not generally well suited for detecting the wide variety of defects that exist in semiconductor wafers and/or reticles.
U.S. Pat. No. 7,295,303 discusses approaches similar to phase-contrast microscopy that are not well suited for detecting a wide variety of defects that exist in semiconductor wafers and/or reticles.
U.S. Pat. No. 7,365,858 and U.S. Application Publication No. 2005/0105097 A1 discuss a system for imaging biological samples. Two modes of operation are discussed, a “phase mode” and an “amplitude mode.” The goal in the discussed amplitude mode is to obtain high contrast raw images. In phase mode, the discussed techniques attempt to extract phase information only. The discussions mention liquid crystal spatial light modulation which is performed in a pupil conjugate through the use of beam splitters and additional lens groups, which are prone to illumination power losses.
U.S. Pat. No. 6,674,522 and U.S. Application Publication No. 2008/0226157 A1 discuss defect detection systems and methods for lithographic masks. They utilize a defocus or Zernike point spread function to detect defects. Their methods are not only complex and require a large amount of computing resources but also not suitable for the detection of small defects.